The present invention generally relates to a method of and an apparatus for synthesizing a diamondlike thin film by using plasma of hydrocarbon gas. Especially, the present invention is characterized in that an excellent diamondlike thin film can be synthesized also on an insulating substrate in the same manner as on an electrically conductive substrate.
A diamondlike thin film has excellent characteristics of hardness, electric resistance, light transmittance, etc. equivalent to those of diamond and is expected to be applied to wear-resistant protective films for various electronic apparatuses or components and passivation films. Various methods of and apparatuses for synthesizing the diamondlike thin film are known. As shown in FIG. 1, an RF plasma CVD apparatus is proposed as one of such synthesis apparatuses by Kito et al. in a Bulletin of Lectures of the 33rd Conference of the Japan Society of Applied Physics, 3p-ZD-14 held in 1986. The prior art RF plasma CVD apparatus includes an upper electrode 117, a lower electrode 120 and a substrate 119 made of SiO.sub.2. The substrate 119 is placed on the lower electrode 120. After a vacuum vessel 122 has been evacuated, benzene gas 118 is introduced into the vacuum vessel 122. Subsequently, when an RF power is applied to the lower electrode 120 by an RF power source 121 of 13.56 MHz, plasma 124, which includes radicals and ions containing carbon, a constituent element of the diamondlike thin film, is produced. By depositing these ions and radicals on the substrate 119, the diamondlike thin film having a Knoop hardness of about 3000 is synthesized on the substrate 119. As will be seen from FIG. 1, when an AC power such as an RF power is applied to the lower electrode having the substrate placed thereon such that plasma of hydrocarbon gas is generated, the diamondlike thin film of high quality can be synthesized not only on a substrate made of electrically conductive material such as metals but on a substrate made of insulating material such as glass.
This can be explained as follows. Namely, when the AC power is applied to plasma, electric potential at which the substrate is lower, in electric potential, than plasma, namely, so-called self-bias potential is produced due to difference in mobility between ions and electrons in plasma. Not only the ions are accelerated towards the substrate so as to be deposited on the substrate but the ions of positive charge deposited on the substrate are neutralized by electrons in plasma, which is a well-known fact.
It is also possible to synthesize the diamondlike thin film by using a known DC plasma CVD apparatus as described by Whitmell et al. in a magazine entitled "Thin Solid Films", Vol. 35 (1976), p. 255-261. However, in the case of the insulating substrate, ions are charged to positive charge on the insulating substrate undesirably. Thus, in order to synthesize the diamondlike thin film on the insulating substrate, it is necessary to further provide a neutralization means such as an electron irradiating means.
The above described known synthesis apparatuses have several drawbacks. One of the drawbacks is that the diamondlike thin film synthesized on the substrate is nonuniform in hardness and thickness. This is because gas flow, gas pressure and plasma state are made nonuniform by the substrate, the electrodes, the vacuum vessel, etc. in the arrangement of the known synthesis apparatus of FIG. 1. Furthermore, since these nonuniformities are affected also by synthesis conditions of the diamondlike thin film such as shape and size of each of the substrate, the electrodes and the vacuum vessel, gas pressure of the vacuum vessel, flow rate of the gas, the applied electric power, etc., it is difficult to eliminate the nonuniformities. Furthermore, in the case where synthesis of the diamondlike thin film is put to practical use industrially by synthesizing the diamondlike thin film on the substrate having a large area, such nonuniformities pose a serious problem.
Another drawback of the known synthesis apparatuses is that abnormal discharge such as arc discharge is likely to take place between the lower electrode and the vacuum vessel in FIG. 1 according to the synthesis conditions in which, for example, the RF power, the gas pressure, etc. are increased in order to raise synthesis speed, so that it becomes difficult to synthesize the diamondlike thin film stably. Namely, since in the arrangement of the known synthesis apparatus of FIG. 1, the gas pressure of the vacuum vessel is substantially identical with pressure in the vicinity of the electrodes for forming the film and the vacuum vessel has ground potential relative to the lower electrode for receiving the electric power, plasma is generated in the vacuum vessel as a whole and thus, arc discharge readily takes place between the lower electrode and substances adhering to the vacuum vessel, projections on the vacuum vessel, etc.
Moreover, since plasma of hydrocarbon gas is generated in the vacuum vessel as a whole as described above, the film adheres also to the wall of the vacuum vessel, a substrate support in the vacuum vessel, etc. As a result, it becomes necessary to perform maintenance operations such as cleaning of the whole vacuum vessel, which is troublesome.
Still another drawback of the known synthesis apparatuses is that when synthesis of the diamondlike thin film is put to practical use industrially by synthesizing the diamondlike thin film on various electronic apparatuses, components, etc., synthesis speed of the diamondlike thin film is unsatisfactorily as low as 100 .ANG./min.
In addition, in the known DC plasma CVD apparatus, it is necessary to provide the electron irradiating means in order to synthesize the diamondlike thin film on the insulating substrate, thereby resulting in not only rise of its production cost but complication of the arrangement and the synthesis operations.
Owing to the above described disadvantages, it has been conventionally difficult to put synthesis of the diamondlike thin film to practical use industrially.